Default to the MarkdownExtra parser
Add FIXMEs
Extend Symfony Filesystem to transform content
Switch to TransformingFilesystem for rendering
Remove renderer and serialiser
Use tags and a compiler pass to register transformers
Ditch SiteBuilder class
Remove obsolete test
Add Twig/HTML transformer
Add a shared Frontmatter parser
Add --delete option
Compile the container
Remove debug code, add required namespace
Add PSR-3 interface
Add logging
Oops, declare log channel
More logging, rename service to logger
Remove ContentCollection and MarkdownFileContentHandler
Remove unused exception
Remove obsolete tests